what is a spell checker?

Wikipedia very succintly defines spell checker as:

In computing terms, a spell checker or spelling checker is a design feature or a software program designed to verify the spelling of words in a document, query, or other context, helping a user to ensure correct spelling. A spell checker may be implemented as a stand-alone application capable of operating on a block of text. Spelling checkers are most often implemented as a feature of a larger application, such as a word processor, email client, electronic dictionary or search engine.

The rest of the article is interesting as well and provides a good overview of spell checkers.